The common cytokine receptor gamma chain and the pre-T cell receptor provide independent but critically overlapping signals in early alpha/beta T cell development

Immunological Phenotyping of Mice with a Point Mutation in Cdk4

Cyclin-dependent kinases (CDKs) play a crucial role in regulation of the mammalian cell cycle. CDK4 and CDK6 control the G1/S restriction checkpoint through their ability to associate with cyclin D proteins in response to growth factor signals. CDK4 deficiency in mice gives rise to a range of endocrine-specific phenotypes including diabetes, infertility, dwarfism, and atrophy of the anterior pituitary. Although CDK6 deficiency can cause thymic atrophy due to a block in the double-negative (DN) to double-positive (DP) stage of T cell development, there are no overt defects in immune cell development reported for CDK4-deficient mice. Here, we examined the impact of a novel N-ethyl-N-nitrosourea-induced point mutation in the gene encoding CDK4 on immune cell development. Mutant mice (Cdk4wnch/wnch) showed normal development and differentiation of major immune cell subsets in the thymus and spleen. Moreover, T cells from Cdk4wnch/wnch mice exhibited normal cytokine production in response to in vitro stimulation. However, analysis of the mixed bone marrow chimeras revealed that Cdk4wnch/wnch-derived T cell subsets and NK cells are at a competitive disadvantage compared to Cdk4+/+-derived cells in the thymus and periphery of recipients. These results suggest a possible role for the CDK4wnch mutation in the development of some immune cells, which only becomes apparent when the Cdk4wnch/wnch mutant cells are in direct competition with wild-type immune cells in the mixed bone marrow chimera.

Immunoregulation via Cell Density and Quorum Sensing-like Mechanisms: An Underexplored Emerging Field with Potential Translational Implications

Quorum sensing (QS) was historically described as a mechanism by which bacteria detect and optimize their population density via gene regulation based on dynamic environmental cues. Recently, it was proposed that QS or similar mechanisms may have broader applications across different species and cell types. Indeed, emerging evidence shows that the mammalian immune system can also elicit coordinated responses on a population level to regulate cell density and function, thus suggesting that QS-like mechanisms may also be a beneficial trait of the immune system. In this review, we explore and discuss potential QS-like mechanisms deployed by the immune system to coordinate cellular-level responses, such as T cell responses mediated via the common gamma chain (γc) receptor cytokines and the aryl hydrocarbon receptors (AhRs). We present evidence regarding a novel role of QS as a multifunctional mechanism coordinating CD4⁺ and CD8⁺ T cell behavior during steady state and in response to infection, inflammatory diseases, and cancer. Successful clinical therapies such as adoptive cell transfer for cancer treatment may be re-evaluated to harness the effects of the QS mechanism(s) and enhance treatment responsiveness. Moreover, we discuss how signaling threshold perturbations through QS-like mediators may result in disturbances of the complex crosstalk between immune cell populations, undesired T cell responses, and induction of autoimmune pathology. Finally, we discuss the potential therapeutic role of modulating immune-system-related QS as a promising avenue to treat human diseases.

Morphologic and Immunohistochemical Characterization of Spontaneous Lymphoma/Leukemia in NSG Mice

The NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ strain (NOD scid gamma, NSG) is a severely immunodeficient inbred laboratory mouse used for preclinical studies because it is amenable to engraftment with human cells. Combining scid and Il2rg^null mutations results in severe immunodeficiency by impairing the maturation, survival, and functionality of interleukin 2-dependent immune cells, including T, B, and natural killer lymphocytes. While NSG mice are reportedly resistant to developing spontaneous lymphomas/leukemias, there are reports of hematopoietic cancers developing. In this study, we characterized the immunophenotype of spontaneous lymphoma/leukemia in 12 NSG mice (20 to 38 weeks old). The mice had a combination of grossly enlarged thymus, spleen, or lymph nodes and variable histologic involvement of the bone marrow and other tissues. All 12 lymphomas were diffusely CD3, TDT, and CD4 positive, and 11 of 12 were also positive for CD8, which together was consistent with precursor T-cell lymphoblastic lymphoma/leukemia (pre-T-LBL). A subset of NSG tissues from all mice and neoplastic lymphocytes from 8 of 12 cases had strong immunoreactivity for retroviral p30 core protein, suggesting an association with a viral infection. These data highlight that NSG mice may develop T-cell lymphoma at low frequency, necessitating the recognition of this spontaneously arising disease when interpreting studies.

Developmental regulation of p53-dependent radiation-induced thymocyte apoptosis in mice

The production of T cell receptor αβ(+) (TCRαβ(+) ) T lymphocytes in the thymus is a tightly regulated process that can be monitored by the regulated expression of several surface molecules, including CD4, CD8, cKit, CD25 and the TCR itself, after TCR genes have been assembled from discrete V, D (for TCR-β) and J gene segments by a site-directed genetic recombination. Thymocyte differentiation is the result of a delicate balance between cell death and survival: developing thymocytes die unless they receive a positive signal to proceed to the next stage. This equilibrium is altered in response to various physiological or physical stresses such as ionizing radiation, which induces a massive p53-dependent apoptosis of CD4(+) CD8(+) double-positive (DP) thymocytes. Interestingly, these cells are actively rearranging their TCR-α chain genes. To unravel an eventual link between V(D)J recombination activity and thymocyte radio-sensitivity, we analysed the dynamics of thymocyte apoptosis and regeneration following exposure of wild-type and p53-deficient mice to different doses of γ-radiation. p53-dependent radio-sensitivity was already found to be high in immature CD4(-) CD8(-) (double-negative, DN) cKit(+) CD25(+) thymocytes, where TCR-β gene rearrangement is initiated. However, TCR-αβ(-) CD8(+) immature single-positive thymocytes, an actively cycling intermediate population between the DN and DP stages, are the most radio-sensitive cells in the thymus, even though their apoptosis is only partially p53-dependent. Within the DP population, TCR-αβ(+) thymocytes that completed TCR-α gene recombination are more radio-resistant than their TCR-αβ(-) progenitors. Finally, we found no correlation between p53 activation and thymocyte sensitivity to radiation-induced apoptosis.

Genetic modification of hematopoietic stem cells as a therapy for HIV/AIDS

The combination of genetic modification and hematopoietic stem cell (HSC) transplantation may provide the necessary means to develop an alternative treatment option to conventional antiretroviral therapy. As HSCs give rise to all hematopoietic cell types susceptible to HIV infection, modification of HSCs is an ideal strategy for the development of infection-resistant immune cell populations. Although promising results have been obtained in multiple animal models, additional evidence is needed to convincingly demonstrate the feasibility of this approach as a treatment of HIV-1 infected patients. Here, we review the potential of HSC transplantation and the recently identified limitations of this approach. Using the Berlin Patient as a model for a functional cure, we contrast the confines of autologous versus allogeneic transplantation. Finally, we suggest that although autologous, gene-modified HSC-transplantation may significantly reduce plasma viremia, reaching the lower detection limits currently obtainable through daily HAART will remain a challenging endeavor that will require innovative combinatorial therapies.

Sequential administration of cytokine genes to enhance cellular immune responses and CD4 (+) T memory cells during DNA vaccination

Antigen specific memory T cells (Tm) have shown to be an important factor in protecting hosts against subsequent infection by previously encountered pathogens. During T-cell activation, several cytokines including IL-6, IL-7 and IL-15, play crucial roles in the development of T cells into memory T cells. With the aim of generating specific Tm, we examined a strategy of sequential administration of molecular adjuvants. In this strategy a DNA vaccine encoding the VP1 capsid protein of foot and mouth disease virus (designated pcD-VP1) was co-delivered to mice along with an IL-6 expressing plasmid (pVAX-IL-6) as an initial molecular adjuvant and boosted with either an IL-7 or IL-15 expressing plasmid, (pVAX-IL-7 or proVAX-IL-15) as the secondary adjuvant. During the pcD-VP1 immunization, we demonstrated that the groups primed with IL-6 and boosted with either IL-7 or IL-15 resulted in the enhancement of cellular and humoral immune responses, maturation of dendritic cells (DCs) and macrophages, and a higher frequency of CD4 (+) Tm (characterized by expressing CD44 (high) CD62L (low) markers, compared with the other groups). Thus, we took advantage of the different effects of cytokines on T cell development, not only to induce a higher level of immune responses after vaccination, but also to generate a higher ratio of CD4 (+) Tm in this sequential cytokine prime-boost study. This would then lead to the mounting of an effective long-term antigen specific immune response.

Conditional deletion of cytokine receptor chains reveals that IL-7 and IL-15 specify CD8 cytotoxic lineage fate in the thymus

The thymus generates T cells with diverse specificities and functions. To assess the contribution of cytokine receptors to the differentiation of T cell subsets in the thymus, we constructed conditional knockout mice in which IL-7Rα or common cytokine receptor γ chain (γ(c)) genes were deleted in thymocytes just before positive selection. We found that γ(c) expression was required to signal the differentiation of MHC class I (MHC-I)-specific thymocytes into CD8(+) cytotoxic lineage T cells and into invariant natural killer T cells but did not signal the differentiation of MHC class II (MHC-II)-specific thymocytes into CD4(+) T cells, even into regulatory Foxp3(+)CD4(+) T cells which require γ(c) signals for survival. Importantly, IL-7 and IL-15 were identified as the cytokines responsible for CD8(+) cytotoxic T cell lineage specification in vivo. Additionally, we found that small numbers of aberrant CD8(+) T cells expressing Runx3d could arise without γ(c) signaling, but these cells were developmentally arrested before expressing cytotoxic lineage genes. Thus, γ(c)-transduced cytokine signals are required for cytotoxic lineage specification in the thymus and for inducing the differentiation of MHC-I-selected thymocytes into functionally mature T cells.

Synergistic effects of interleukin-7 and pre-T cell receptor signaling in human T cell development

The role of IL-7 in pre-T cell receptor (TCR) signaling during human T cell development is poorly understood. To study this, we engineered Molt3, a T cell progenitor T-acute lymphoblastic leukemia cell line, using lentiviral IL-7 receptor α (IL-7Rα) to serve as a model system. IL-7 promoted pre-TCR activation in IL-7Rα(hi) Molt3 as illustrated by CD25 up-regulation after anti-CD3 stimulation. Anti-CD3 treatment activated Akt and Erk1/2 signaling pathways as proven using specific inhibitors, and IL-7 further enhanced both signaling pathways. The close association of IL-7Rα with CD3ζ in the pre-TCR complex was illustrated through live imaging confocal fluorescence microscopy. These results demonstrate a direct and cooperative role of IL-7 in pre-TCR signaling.

Thymocytes may persist and differentiate without any input from bone marrow progenitors

Thymus transplants can correct deficiencies of the thymus epithelium caused by the complete DiGeorge syndrome or FOXN1 mutations. However, thymus transplants were never used to correct T cell-intrinsic deficiencies because it is generally believed that thymocytes have short intrinsic lifespans. This notion is based on thymus transplantation experiments where it was shown that thymus-resident cells were rapidly replaced by progenitors originating in the bone marrow. In contrast, here we show that neonatal thymi transplanted into interleukin 7 receptor-deficient hosts harbor populations with extensive capacity to self-renew, and maintain continuous thymocyte generation and export. These thymus transplants reconstitute the full diversity of peripheral T cell repertoires one month after surgery, which is the earliest time point studied. Moreover, transplantation experiments performed across major histocompatibility barriers show that allogeneic transplanted thymi are not rejected, and allogeneic cells do not induce graft-versus-host disease; transplants induced partial or total protection to infection. These results challenge the current dogma that thymocytes cannot self-renew, and indicate a potential use of neonatal thymus transplants to correct T cell-intrinsic deficiencies. Finally, as found with mature T cells, they show that thymocyte survival is determined by the competition between incoming progenitors and resident cells.

Thymic epithelial cells: the multi-tasking framework of the T cell "cradle"

The thymus provides the anatomical "cradle" that fosters developing thymocytes. Thymic epithelial cells (TECs) are specialized cellular components that may be viewed as a multifunctional "frame" to nurture distinct stages of thymopoiesis. A symbiotic relationship between TECs and thymocytes exists because reciprocal interactions are required to achieve complete maturation of both cell types. Here, we propose that crucial instructive signals delivered by developing thymocytes negatively regulate functional attributes of immature TECs (including the expression of Delta-like 4 (DLL4) and interleukin-7 (IL-7)) that are required during early stages of thymopoiesis, while promoting the diversification of more mature TEC subsets. Thus, the division of labour among TECs may be coordinated directly by local cellular feedback mechanisms operating within distinct thymic niches.

CSL-MAML-dependent Notch1 signaling controls T lineage-specific IL-7R{alpha} gene expression in early human thymopoiesis and leukemia

Notch1 activation is essential for T-lineage specification of lymphomyeloid progenitors seeding the thymus. Progression along the T cell lineage further requires cooperative signaling provided by the interleukin 7 receptor (IL-7R), but the molecular mechanisms responsible for the dynamic and lineage-specific regulation of IL-7R during thymopoiesis are unknown. We show that active Notch1 binds to a conserved CSL-binding site in the human IL7R gene promoter and critically regulates IL7R transcription and IL-7R α chain (IL-7Rα) expression via the CSL–MAML complex. Defective Notch1 signaling selectively impaired IL-7Rα expression in T-lineage cells, but not B-lineage cells, and resulted in a compromised expansion of early human developing thymocytes, which was rescued upon ectopic IL-7Rα expression. The pathological implications of these findings are demonstrated by the regulation of IL-7Rα expression downstream of Notch1 in T cell leukemias. Thus, Notch1 controls early T cell development, in part by regulating the stage- and lineage-specific expression of IL-7Rα.

Sustained expression of pre-TCR induced beta-catenin in post-beta-selection thymocytes blocks T cell development

Pre-TCR and IL-7R signals regulate beta-selection of thymocytes and then must be down-regulated for further development. However, the molecular events that control down-regulation remain unknown. We and others have previously shown that beta-catenin in cooperation with TCF regulates beta-selection. In this paper, we demonstrate that beta-catenin expression is stringently regulated by intrathymic signals, it is expressed at the highest levels in the pre-TCR signaled thymocytes, and is down-regulated in post-beta-selection thymocytes. Pre-TCR-induced beta-catenin regulates initial stages of pre-TCR signaling including expression of early growth response (Egr) genes but must be down-regulated to express RORgammat, which is essential for maturation to the CD4+CD8+ double positive (DP) stage. Sustained expression of beta-catenin results in the generation of IL-7R-, Egr-, and TGFbeta-expressing pre-DP thymocytes that are blocked in development. These data are consistent with a model in which post-beta-selection, pre-TCR-induced beta-catenin expression must return to background levels for efficient transition to the DP stage.

Characterization of the thymic IL-7 niche in vivo

The thymus represents the "cradle" for T cell development, with thymic stroma providing multiple soluble and membrane cues to developing thymocytes. Although IL-7 is recognized as an essential factor for thymopoiesis, the "environmental niche" of thymic IL-7 activity remains poorly characterized in vivo. Using bacterial artificial chromosome transgenic mice in which YFP is under control of IL-7 promoter, we identify a subset of thymic epithelial cells (TECs) that co-express YFP and high levels of Il7 transcripts (IL-7(hi) cells). IL-7(hi) TECs arise during early fetal development, persist throughout life, and co-express homeostatic chemokines (Ccl19, Ccl25, Cxcl12) and cytokines (Il15) that are critical for normal thymopoiesis. In the adult thymus, IL-7(hi) cells localize to the cortico-medullary junction and display traits of both cortical and medullary TECs. Interestingly, the frequency of IL-7(hi) cells decreases with age, suggesting a mechanism for the age-related thymic involution that is associated with declining IL-7 levels. Our temporal-spatial analysis of IL-7-producing cells in the thymus in vivo suggests that thymic IL-7 levels are dynamically regulated under distinct physiological conditions. This IL-7 reporter mouse provides a valuable tool to further dissect the mechanisms that govern thymic IL-7 expression in vivo.

Expression profiling of immature thymocytes revealed a novel homeobox gene that regulates double-negative thymocyte development

Intrathymic development of CD4/CD8 double-negative (DN) thymocytes can be tracked by well-defined chronological subsets of thymocytes, and is an ideal target of gene expression profiling analysis to clarify the genetic basis of mature T cell production, by which differentiation of immature thymocytes is investigated in terms of gene expression profiles. In this study, we show that development of murine DN thymocytes is predominantly regulated by largely repressive rather than inductive activities of transcriptions, where lineage-promiscuous gene expression in immature thymocytes is down-regulated during their differentiation. Functional mapping of genes showing common temporal expression profiles implicates previously uncharacterized gene regulations that may be relevant to early thymocytes development. A small minority of genes is transiently expressed in the CD44(low)CD25(+) subset of DN thymocytes, from which we identified a novel homeobox gene, Duxl, whose expression is up-regulated by Runx1. Duxl promotes the transition from CD44(high)CD25(+) to CD44(low)CD25(+) in DN thymocytes, while constitutive expression of Duxl inhibits expression of TCR beta-chains and leads to impaired beta selection and greatly reduced production of CD4/CD8 double-positive thymocytes, indicating its critical roles in DN thymocyte development.

gamma(c) cytokines provide multiple homeostatic signals to naive CD4(+) T cells

Cytokines signaling through receptors sharing the common gamma chain (gamma(c)), including IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21, are critical for the generation and peripheral homeostasis of B, T and NK cells. To identify unique or redundant roles for gamma(c) cytokines in naive CD4(+) T cells, we compared monoclonal populations of CD4(+) T cells from TCR-Tg mice that were gamma(c) (+), gamma(c) (-), CD127(-/-) or CD122(-/-). We found that gamma(c) (-) naive CD4(+) T cells failed to accumulate in the peripheral lymphoid organs and the few remaining cells were characterized by small size, decreased expression of MHC class I and enhanced apoptosis. By over-expressing human Bcl-2, peripheral naive CD4(+) T cells that lack gamma(c) could be rescued. Bcl-2(+) gamma(c) (-) CD4(+) T cells demonstrated enhanced survival characteristics in vivo and in vitro, and could proliferate normally in vitro in response to antigen. Nevertheless, Bcl-2(+) gamma(c) (-) CD4(+) T cells remained small in size, and this phenotype was not corrected by enforced expression of an activated protein kinase B. We conclude that gamma(c) cytokines (primarily but not exclusively IL-7) provide Bcl-2-dependent as well as Bcl-2-independent signals to maintain the phenotype and homeostasis of the peripheral naive CD4(+) T cell pool.

IL-7/STAT5 cytokine signaling pathway is essential but insufficient for maintenance of naive CD4 T cell survival in peripheral lymphoid organs

Constitutive expression of suppressors of cytokine signaling (SOCS)1 in T lineage in vivo attenuated cytokine signaling and resulted in a dramatic reduction in the number of naive CD44(low)CD62L(high) CD4 T cells in the spleen. After adoptive transfer of thymocytes from SOCS1 transgenic mice into normal recipients, naive CD4 T cells rapidly disappeared from the spleen within 1 wk. Likewise, T cell-specific deletion of STAT5a/b in vivo resulted in a similar phenotype characterized by loss of naive CD4 T cells. Thus, STAT5-mediated signaling is crucial for promoting naive T cell survival. However, forced expression of constitutively active STAT5 failed to rescue CD4 T cells in SOCS1 transgenic mice, implying that STAT5 activation is necessary but not sufficient for naive CD4 T cell survival. Although blockade of the IL-7R, a SOCS1 target, resulted in clear inhibition of naive T cell survival, the effect occurred 3 wk after anti-IL-7R Ab treatment, but not at earlier time points. These results suggest that IL-7-mediated STAT5 activation is essential for long-term survival of naive CD4 cells after export from thymus, and that another SOCS1-sensitive cytokine is critical for short-term naive T cell survival.

Neuropilin 1 and CD25 co-regulation during early murine thymic differentiation

Neuropilin 1 (NP1) is a receptor for both semaphorin and vascular endothelial growth factor expressed by subpopulations of neuronal and endothelial cells. In the immune system, NP1 is present on dendritic and regulatory T cells. Here, we show that NP1 is expressed in the murine thymus, starting on day 12.5 of gestation. In the adult, NP1 is mainly expressed by CD4(-)CD8(-) double negative cells, CD4+CD8+ double positive cells, and CD4+CD25+ regulatory T cells but barely detected in single CD4+ and CD8+ positive thymocytes. Within the CD4(-)CD8(-)CD3(-) (triple-negative, TN) immature cells, NP1 expression starts in TN3 (CD44(-)CD25+) and increases in TN4 (CD44(-)CD25(-)) cells. In order to study the role of NP1 in thymocyte differentiation, we generated mice in which the np1 gene is selectively disrupted in the T-cell lineage. The mutant mice display normal thymocyte, peripheral, conventional and CD4+CD25+Foxp3+ regulatory T-cell populations. However, we observe a down-regulation of the CD25 expression between the TN3 and TN4 stages that is (i) correlated to increased expression of NP1 in control mice and (ii) altered in mutant mice, suggesting that NP1 is co-regulated with CD25 during early immature thymocyte differentiation.

Pre-TCR expression cooperates with TEL-JAK2 to transform immature thymocytes and induce T-cell leukemia

The TEL-JAK2 gene fusion, which has been identified in human leukemia, encodes a chimeric protein endowed with constitutive tyrosine kinase activity. TEL-JAK2 transgenic expression in the mouse lymphoid lineage results in fatal and rapid T-cell leukemia/lymphoma. In the present report we show that T-cell leukemic cells from EmuSRalpha-TEL-JAK2 transgenic mice present an aberrant CD8(+) differentiation phenotype, as determined by the expression of stage-specific cell surface markers and lineage-specific genes. TEL-JAK2 transforms immature CD4(-)CD8(-) double-negative thymocytes, as demonstrated by the development of T-cell leukemia with full penetrance in a Rag2-deficient genetic background. This disease is similar to the bona fide TEL-JAK2 disease as assessed by phenotypic and gene profiling analyses. Pre-TCR signaling synergizes with TEL-JAK2 to transform immature thymocytes and initiate leukemogenesis as shown by (1) the delayed leukemia onset in Rag2-, CD3epsilon- and pTalpha-deficient mice, (2) the occurrence of recurrent chromosomal alterations in pre-TCR-deficient leukemia, and (3) the correction of delayed leukemia onset in Rag2-deficient TEL-JAK2 mice by an H-Y TCRalphabeta transgene that mimics pre-TCR signaling. Although not affecting leukemia incidence and mouse survival, TCRalphabeta expression was shown to facilitate leukemic cell expansion in secondary lymphoid organs.

Antigen receptor regulation of phosphoinositide-dependent kinase 1 pathways during thymocyte development

Phosphoinositide-dependent kinase 1 (PDK1) is essential for T cell development but little is know about the stimuli that regulate PDK1 signaling in vivo. The thymus contains a heterogeneous mixture of cells at different stages of development making it difficult to use biochemical techniques to examine the activity of PDK1 pathways as thymocytes develop in situ. Herein, we use a single cell assay to quantify activation of the PDK1 target kinase ribosomal S6 kinase 1 (S6K1) in different murine thymocyte subsets immediately ex vivo. This technique allows an assessment of S6K1 activation as thymocytes respond to developmental stimuli in vivo. These studies reveal that only a small percentage of thymocytes show evidence for activation of PDK1 mediated signaling in situ. The thymic subpopulations that contain active PDK1/S6K1 are those known to be responding to signaling by the pre T cell receptor and the mature alpha/beta T cell antigen receptor (TCR). Moreover, loss of antigen receptor signaling in T cell progenitors that cannot rearrange their TCR beta locus prevents in vivo activation of S6K1. The present data identifying antigen receptor signaling as a key activator of PDK1 mediated signaling afford a molecular explanation for the important role of this molecule in T cells.

Progression of regulatory gene expression states in fetal and adult pro-T-cell development

Precursors entering the T-cell developmental pathway traverse a progression of states characterized by distinctive patterns of gene expression. Of particular interest are regulatory genes, which ultimately control the dwell time of cells in each state and establish the mechanisms that propel them forward to subsequent states. Under particular genetic and developmental circumstances, the transitions between these states occur with different timing, and environmental feedbacks may shift the steady-state accumulations of cells in each state. The fetal transit through pro-T-cell stages is faster than in the adult and subject to somewhat different genetic requirements. To explore causes of such variation, this review presents previously unpublished data on differentiation gene activation in pro-T cells of pre-T-cell receptor-deficient mutant mice and a quantitative comparison of the profiles of transcription factor gene expression in pro-T-cell subsets of fetal and adult wildtype mice. Against a background of consistent gene expression, several regulatory genes show marked differences between fetal and adult expression profiles, including those encoding two basic helix-loop-helix antagonist Id factors, the Ets family factor SpiB and the Notch target gene Deltex1. The results also reveal global differences in regulatory alterations triggered by the first T-cell receptor-dependent selection events in fetal and adult thymopoiesis.

Decreased CD127 expression on T Cells in HIV-1-infected adults receiving antiretroviral therapy with or without intermittent IL-2 therapy

BACKGROUND

The interleukin-7 (IL-7)/IL-7 receptor alpha (IL-7Ralpha) system is an important regulator of T-cell homeostasis. We evaluated the IL-7/IL-7Ralpha system in a large cohort of HIV-infected patients, including a subset treated with intermittent IL-2.

METHODS

IL-7 serum levels and CD127 (IL-7Ralpha) expression on T cells were evaluated in a cross-sectional study of 36 healthy volunteers, 151 HIV-infected patients, and 83 HIV-infected patients who had received IL-2 therapy. Multivariate regression models were used to determine predictors of CD127 expression.

RESULTS

HIV-infected patients had higher IL-7 levels compared with healthy volunteers (P = 0.022) and IL-2-treated patients (P = 0.012). CD127 expression was significantly lower on CD4 and CD8 T cells of HIV-infected patients compared with healthy volunteers (P = 0.008 and P < 0.001, respectively), and CD127 median fluorescence intensity was lowest on CD4 T cells in IL-2-treated patients (P < 0.001 compared with HIV-infected patients). The proportion of naive and effector memory/effector T cells were significant predictors of CD127 expression on T cells. IL-2 immunotherapy led to the expansion of a CD25/CD127-low subset of CD4 T cells.

CONCLUSIONS

CD127 expression on T cells remains low in HIV-infected patients despite antiretroviral therapy, reflecting persistent aberration in the subset composition of the T-cell pool.

T cell development, ageing and Interleukin-7

Interleukin-7 (IL-7) is a cytokine with a central role in the development and maintenance of the peripheral T cell pool. In the mouse, expression of the IL-7 gene in the thymus has been carefully followed from gestation onwards throughout the lifespan. One of the features of its expression in the thymus is that it changes with time, declining measurably as the animal ages. This reduction is associated with a decrease in thymic size, cellularity and output. Analysis of transgenic animals carrying either IL-7 or IL-7 receptor transgenes reveals that the intrathymic level of IL-7 has a critical effect on the production of T cells, and that this may not be a linear relationship. This is an important consideration for therapy involving treatment of old animals with IL-7 of which there are reports indicating some rejuvenation of the thymus following IL-7 treatment, which is never complete. The thymus does not appear to return to the size and cellularity seen in youth. Several possible scenarios could account for this, including the inability to maintain IL-7 within defined limits in the thymus during the therapy.

Constitutive expression of the pre-TCR enables development of mature T cells

Expression and signalling through the pre-TCR and the TCRalphabeta resemble two critical checkpoints during T cell development. We investigated to which extent a pre-TCR can functionally replace mature TCRalpha chains during T cell development. For this purpose, transgenic mice were generated expressing the pre-TCRalpha (pTalpha) under the transcriptional control of TCRbeta regulatory elements. We report here on the interesting finding that constitutive pTalpha expression allows complete T cell maturation. The pre-TCR complex permits a subset of beta-selected thymocytes to mature in the absence of TCRalpha into peripheral T cells (betaT cells) comprising up to 10% of all lymphocytes. Lymphopenia-driven proliferation of these betaT cells is similar to that of conventional alphabetaT cells. Furthermore, betaT cells proliferated and acquired effector function upon stimulation with allogeneic MHC.

A unique function for cyclin D3 in early B cell development

During hematopoiesis, stem cell proliferation is dependent on expression of the D-type cyclins. However, little is known about how each cyclin D contributes to the development of specific hematopoietic lineages. Here, analysis of Ccnd1(-/-), Ccnd2(-/-), Ccnd3(-/-) and Ccnd2(-/-)Ccnd3(-/-) mice showed that cyclin D3 was uniquely required for the development of pre-B cells. Transcription of Ccnd3 was dependent on expression of the common gamma-chain. In contrast, expression of the pre-B cell receptor and activation of 'downstream' signaling pathways prevented proteasome-mediated degradation of cyclin D3. Cyclin D3 has a key function in B cell development by integrating cytokine and pre-B cell receptor-dependent signals to expand the pool of pre-B cells that have successfully rearranged immunoglobulin heavy chain.

Regulation of T-cell progenitor survival and cell-cycle entry by the pre-T-cell receptor

Pre-T-cell receptor (pre-TCR) functions and the study of early thymocyte development continue to fascinate immunologists more than 10 years after the first description and cloning of the receptor. Although multiple reports have addressed several aspects of pre-TCR signaling and function, its ability to regulate diverse functions, including proliferation, survival, and allelic exclusion of the TCR-beta locus, remains an open question. What fascinates us is its central role in the fine balance between physiological differentiation and thymocyte transformation that leads to T-cell leukemia and lymphomas. In this review, we integrate pre-TCR signaling pathways and study their effects on the regulation of T-cell progenitor cell-cycle entry and cell survival. We also connect aberrant pre-TCR signaling to deregulated proliferation and apoptotic balances and thymocyte transformation.

The pre-B cell receptor in B cell development: recent advances, persistent questions and conserved mechanisms

B cell development is a process tightly regulated by the orchestrated signaling of cytokine receptors, the pre-B cell receptor (BCR) and the B cell receptor (BCR). It commences with common lymphoid progenitors (CLP) up-regulating the expression of B cell-related genes and committing to the B cell lineage. Cytokine signaling (IL-7, stem cell factor, FLT3-L) is essential at this stage of development as it suppresses cell death, sustains proliferation and facilitates heavy chain rearrangements. As a result of heavy chain recombination, the pre-BCR is expressed, which then becomes the primary determiner of survival, cell cycle entry and allelic exclusion. In this review, we discuss the mechanisms of B cell lineage commitment and describe the signaling pathways that are initiated by the pre-BCR. Finally, we compare pre-BCR and pre-TCR structure, signal transduction and function, drawing parallels between early pre-B and pre-T cell development.

Tid1 Is Required for T Cell Transition from Double-Negative 3 to Double-Positive Stages

Tid1, a DnaJ cochaperone protein, is the mammalian homologue of the Drosophila tumor suppressor Tid56 whose antitumor function is most likely mediated through its capacity to regulate cell differentiation in imaginal discs. We suspected that the mammalian counterpart, tid1, may also be involved in regulating cell differentiation. To investigate this, we exploited the system of T cell development to examine whether tid1 plays a role in this well-defined process. Mice with tid1 specifically deleted in T cells developed thymic atrophy, with dramatic reduction of double-positive and single-positive thymocytes in the tid1(-/-) thymus. Although the subpopulations of tid1(-/-) double-negative (DN) 1-3 thymocytes were normal, the subpopulation of DN4 thymocytes was measurably smaller because of reduced proliferation and significant cell death. Immature tid1(-/-) thymocytes show normal VDJ beta-chain rearrangement and pre-TCR and CD3 expression in both DN3 and DN4 thymocytes, but in DN4 thymocytes, there was significantly reduced expression of the antiapoptotic bcl-2 gene. Restoring the expression level of Bcl-2 protein in tid1(-/-) thymus by introduction of a transgenic human bcl-2 gene resulted in reversal of the developmental defects in tid1(-/-) thymus. Together, these results demonstrate that tid1 is critical in early thymocyte development, especially during transition from the DN3 to double-positive stages, possibly through its regulation of bcl-2 expression, which provides survival signals.

Interleukin-7 signalling is sufficient to phenotypically and functionally prime human CD4 naive T cells

Interleukin-7 (IL-7) is produced by bone marrow and lymphoid stromal cells and is involved in the synthesis, survival and homeostasis of T cells. These attributes are the basis for current strategies to utilize IL-7 as an immune modulator for several clinical conditions to replenish depleted T-cell numbers. Because we had previously determined that IL-7 can induce potent human immunodeficiency virus replication in the otherwise non-permissive CD4(+) naive T-cell compartment, we evaluated here the impact of IL-7 on the phenotype and functional potential of naive CD4(+) T cells in an attempt to understand the mechanism of this induction. We demonstrate that IL-7 mediated the up-regulation of CD25, CD95 and human leucocyte antigen-DR, while it did not alter the expression of CD45RO, CD69, CD40, or CD154. Examination of the cytokine profile of IL-7-treated naive T cells using a Type1/Type2 Proteome Array indicated a remarkable IL-7-mediated induction of interferon-gamma production, while the other cytokines evaluated (IL-2, IL-12, tumour necrosis factor-alpha, IL-4, IL-5, IL-10 and IL-13) were not affected. Intracellular staining of IL-7-treated naive T cells for interferon-gamma verified the Proteome data. IL-7 did not induce cell cycle proliferation of naive CD4(+) T cells, as evaluated by 7-AAD/pyronin immunostaining and carboxyfluorescein diacetate succinimidyl ester dye tracking. IL-7 treatment of naive CD4(+) T cells induced their ability to prime monocytes, as was indicated by induction of CD80 and CD86 expression on monocytes cocultured with IL-7-treated naive CD4(+) T cells. Collectively, these data indicate that IL-7 signalling is sufficient to phenotypically and functionally prime human CD4(+) naive T cells independent of antigen stimulation.

Deranged Early T Cell Development in Immunodeficient Strains of Nonobese Diabetic Mice

NOD mice exhibit defects in T cell functions that have been postulated to contribute to diabetes susceptibility in this strain. However, early T cell development in NOD mice has been largely unexplored. NOD mice with the scid mutation and Rag1 deficiency were analyzed for pre-T cell development in the NOD genetic background. These strains reveal an age-dependent, programmed breakdown in beta selection checkpoint enforcement. At 5-8 wk of age, even in the absence of TCRbeta expression, CD4+ and CD4+CD8+ blasts appear spontaneously. However, these breakthrough cells fail to restore normal thymic cellularity. The breakthrough phenotype is recessive in hybrid (NODxB6)F1-scid and -Rag1null mice. The breakthrough cells show a mosaic phenotype with respect to components of the beta selection program. They mimic normal beta selection by up-regulating germline TCR-Calpha transcripts, CD2, and Bcl-xL and down-regulating Bcl-2. However, they fail to down-regulate transcription factors HEB-alt and Hes1 and initially express aberrantly high levels of Spi-B, c-kit (CD117), and IL-7Ralpha. Other genes examined distinguish this form of breakthrough from previously reported models. Some of the abnormalities appear first in a cohort of postnatal thymocytes as early as the double-negative 2/double-negative 3 transitional stage. Thus, our results reveal an NOD genetic defect in T cell developmental programming and checkpoint control that permits a subset of the normal outcomes of pre-TCR signaling to proceed even in the absence of TCRbeta rearrangement. Furthermore, this breakthrough may initiate thymic lymphomagenesis that occurs with high frequency in both NOD-scid and -Rag1null mice.

IL-7 receptor signals inhibit expression of transcription factors TCF-1, LEF-1, and RORgammat: impact on thymocyte development

Intrathymic T cell development depends on signals transduced by both T cell receptor and cytokine receptors. Early CD4⁻CD8⁻ (double negative) thymocytes require interleukin (IL)-7 receptor (IL-7R) signals for survival and proliferation, but IL-7R signals are normally extinguished by the immature single positive (ISP) stage of thymocyte development. We now demonstrate that IL-7R signals inhibit expression of transcription factors TCF-1, LEF-1, and RORγt that are required for the ISP to double positive (DP) transition in the thymus. In addition, we demonstrate that IL-7R signals also inhibit TCF-1 and LEF-1 expression in mature peripheral T cells. Thus, the present work has identified several important downstream target genes of IL-7R signaling in T cells and thymocytes that provide a molecular mechanism for the inhibitory influence of IL-7R signaling on DP thymocyte development. We conclude that IL-7R signals down-regulate transcription factors required for the ISP to DP transition and so must be terminated by the ISP stage of thymocyte development.

Regulation of Thymus Size by Competition for Stromal Niches among Early T Cell Progenitors

Thymic T cell production is characterized by differentiating waves of non-self-renewing, bone marrow-derived progenitors. The factors constraining new progenitor recruitment, intrathymic precursor expansion, and thymus size remain enigmatic, but are believed to be controlled by a feedback loop responding to lymphoid cellularity and competition for stromal niches. In this study, we show that competition for stromal niches does occur, but is solely limited to cells at the early CD4(-)8(-) precursor stages of differentiation. The overall size of the organ is determined both by this limitation on early precursor expansion, and by a second, cell-intrinsic limit on expansion of progenitor cells transiting to the CD4(+)8(+) stage. Together with asymmetric use of marrow-derived progenitors to reconstitute the intrathymic pool, these processes facilitate continuous generation of new T cells while maintaining a relatively stable organ size.

Interleukin-7: An Interleukin for Rejuvenating the Immune System

Infection of an individual (aged 20-30 years) by a virus will cause a response from the T (thymus derived) lymphocytes of which there are approximately 3 x 10(11). If the individual has not met the virus before, the response will come from the naive T cell subset (50 +/- 10% of the total T cell pool at this age) containing recent thymic emigrants produced from the thymus at approximately 10(8) per day. Their antigen-specific receptor has a defined specificity governed by the conformation of its two chains (alpha and beta), and the repertoire of specificities is somewhere in the region of 2 x 10(7) to 10(8). A successful response leads to clonal expansion and the generation of memory T cells to the infecting agent.

Deletion of β-catenin impairs T cell development

T cells encounter two main checkpoints during development in the thymus. These checkpoints are critically dependent on signals derived from the thymic microenvironment as well as from the pre-T cell receptor (pre-TCR) and the alphabeta TCR. Here we show that T cell-specific deletion of beta-catenin impaired T cell development at the beta-selection checkpoint, leading to a substantial decrease in splenic T cells. In addition, beta-catenin also seemed to be a target of TCR-CD3 signals in thymocytes and mature T cells. These data indicate that beta-catenin-mediated signals are required for normal T cell development.

Signal-transducing adaptor molecules STAM1 and STAM2 are required for T-cell development and survival

We previously reported that the STAM family members STAM1 and STAM2 are phosphorylated on tyrosine upon stimulation with cytokines through the gammac-Jak3 signaling pathway, which is essential for T-cell development. Mice with targeted mutations in either STAM1 or STAM2 show no abnormality in T-cell development, and mice with double mutations for STAM1 and STAM2 are embryonically lethal; therefore, here we generated mice with T-cell-specific double mutations for STAM1 and STAM2 using the Cre/loxP system. These STAM1(-/-) STAM2(-/-) mice showed a significant reduction in thymocytes and a profound reduction in peripheral mature T cells. In proliferation assays, thymocytes derived from the double mutant mice showed a defective response to T-cell-receptor (TCR) stimulation by antibodies and/or cytokines, interleukin-2 (IL-2) and IL-7. However, signaling events downstream of receptors for IL-2 and IL-7, such as activations of STAT5, extracellular signal-regulated kinase (ERK), and protein kinase B (PKB)/Akt, and c-myc induction, were normal in the double mutant thymocytes. Upon TCR-mediated stimulation, prolonged activations of p38 mitogen-activated protein kinase and Jun N-terminal protein kinase were seen, but activations of ERK, PKB/Akt, and intracellular calcium flux were normal in the double mutant thymocytes. When the cell viability of cultured thymocytes was assessed, the double mutant thymocytes died more quickly than controls. These results demonstrate that the STAMs are indispensably involved in T-cell development and survival in the thymus through the prevention of apoptosis but are dispensable for the proximal signaling of TCR and cytokine receptors.

A positive look at double-negative thymocytes

In some respects, our understanding of the cellular and molecular aspects of early T-cell differentiation is lagging behind that of B cells. Papers describing gene-knockout and reporter-transgenic mice in which thymocyte development is affected are often difficult to interpret. Progress in this field will be hampered unless a more detailed phenotypic and molecular analysis of progenitor thymocytes at the single-cell level is carried out.

Definition of regulatory network elements for T cell development by perturbation analysis with PU.1 and GATA-3

PU.1 and GATA-3 are transcription factors that are required for development of T cell progenitors from the earliest stages. Neither one is a simple positive regulator for T lineage specification, however. When expressed at elevated levels at early stages of T cell development, each of these transcription factors blocks T cell development within a different, characteristic time window, with GATA-3 overexpression initially inhibiting at an earlier stage than PU.1. These perturbations are each associated with a distinct spectrum of changes in the regulation of genes needed for T cell development. Both transcription factors can interfere with expression of the Rag-1 and Rag-2 recombinases, while GATA-3 notably blocks PU.1 and IL-7Ralpha expression, and PU.1 reduces expression of HES-1 and c-Myb. A first-draft assembly of the regulatory targets of these two factors is presented as a provisional gene network. The target genes identified here provide insight into the basis of the effects of GATA-3 or PU.1 overexpression and into the regulatory changes that distinguish the developmental time windows for these effects.

Interleukin-7 inhibits pre-T-cell differentiation induced by the pre-T-cell receptor signal and the effect is mimicked by hGM-CSF in hGM-CSF receptor transgenic mice

We have previously reported that human granulocyte-macrophage colony-stimulating factor (hGM-CSF) causes a stage-specific inhibition of T-cell receptor (TCR) alphabeta cell development in the thymus of transgenic mice constitutively expressing the hGM-CSF receptor. Since it has been reported that the addition of interleukin-7 (IL-7) to fetal thymic organ culture (FTOC) has similar effects, we compared the effects of IL-7 and hGM-CSF on TCR(alphabeta) cell development in hGM-CSF receptor transgenic mice. We reconstituted fetal lobes with sorted pre-T, or post pre-T CD4(-)CD8(-) precursor cells. The addition of either IL-7 or hGM-CSF to these cultures suppressed further differentiation of pre-T cells but not post pre-T cells. At the same time, the cell number was increased, suggesting that pre-T-cell proliferation is stimulated by these cytokines. Furthermore, the differentiation of recombination-activating gene-1 (RAG-1)-deficient pre-T cells in response to anti-CD3 antibody stimulation was suppressed by either IL-7 or hGM-CSF, suggesting that these cytokines inhibit the pre-T-cell receptor (pre-TCR) signal. This inhibition is unexpected because the pre-TCR signal and the IL-7 signal have previously been considered to be co-operative. Recent analysis of the downstream events of IL-7 receptor and GM-CSF receptor revealed that they share common signal transduction molecules. Our results show that IL-7 is able to promote pre-T cell proliferation and to suppress differentiation induced by the pre-TCR signal. GM-CSF can mimic these biological activities of IL-7 when the pre-T cells express GM-CSF receptors. Our data suggest that both timing and level of activation of the IL-7 signalling pathway must be precisely regulated to facilitate the differentiation of thymocytes.

Interleukin-7-treated naive T cells can be productively infected by T-cell-adapted and primary isolates of human immunodeficiency virus 1

Although human immunodeficiency virus (HIV) gag/pol DNA can be detected in naive T cells, whether naive T cells can be productively infected by HIV is still questionable. Given that interleukin-7 (IL-7) is a prospective therapeutic immunomodulator for the treatment of HIV, we evaluated the effect of IL-7 on promoting naive T-cell infection of laboratory-adapted (IIIB), M-tropic, and primary isolates of HIV. Initially, we determined that the 3 cell surface markers widely used to identify naive T cells (CD45RA(+)CD45RO(-), CD45RA(+)CD62L(+), and CD45RO(-)CD27(+)CD95(low)) are all equivalent in T-cell receptor excision circle content, a marker for the replicative history of a cell as well as for de novo T cells. We therefore used CD45RA(+)CD45RO(-) expression to define naive T cells in this study. We demonstrate that although untreated or IL-2-treated naive T cells are not productively infected by HIV, IL-7 pretreatment mediated the productive infection of laboratory-adapted, M-tropic, and primary isolates of HIV as determined by p24 core antigen production. This up-regulation was between 8- and 58-fold, depending on the HIV isolate used. IL-7 pretreatment of naive T cells also potently up-regulated surface expression of CXCR4 but not CCR5 and mediated the expansion of naive T cells without the acquisition of the primed CD45RO phenotype. Collectively, these data indicate that IL-7 augments naive T-cell susceptibility to HIV and that under the appropriate environmental milieu, naive T cells may be a source of HIV productive infection. This information needs to be considered in evaluating IL-7 as an immunomodulator for HIV-infected patients.

Thymocyte depletion in Trypanosoma cruzi infection is mediated by trans-sialidase-induced apoptosis on nurse cells complex

Trypanosoma cruzi, the causative agent of Chagas' disease, induces transient thymic aplasia early after infection-a phenomenon that still lacks a molecular explanation. The parasite sheds an enzyme known as trans-sialidase (TS), which is able to direct transfer-sialyl residues among macromolecules. Because cell-surface sialylation is known to play a central role in the immune system, we tested whether the bloodstream-borne TS is responsible for the thymic alterations recorded during infection. We found that recombinant TS administered to naive mice was able to induce cell-count reduction mediated by apoptosis, mimicking cell subsets distribution and histologic findings observed during the acute phase of the infection. Thymocytes taken after TS treatment showed low response to Con A, although full ability to respond to IL-2 or IL-2 plus Con A was conserved, which resembles findings from infected animals. Alterations were found to revert several days after TS treatment. The administration of TS-neutralizing Abs to T. cruzi-infected mice prevented thymus alterations. Results indicate that the primary target for the TS-induced apoptosis is the so-called "nurse cell complex". Therefore, we report here supporting evidence that TS is the virulence factor from T. cruzi responsible for the thymic alterations via apoptosis induction on the nurse cell complex, and that TS-neutralizing Abs elicitation during infection is associated with the reversion to thymic normal parameters.

A pTalpha-negative subpopulation of CD25+ TN thymocytes revealed by a transgenic marker

We have recently generated 5'lambda5-huTAC mice, which express the human CD25 (huTAC) gene under the control of the 5'-flanking region of the mouse lambda5-gene. The huTAC-transgene was expressed in pre-B cells but neither in mature B cells nor in T cells of these mice. In this report we demonstrate that the transgene is also transiently expressed by adult CD25+ CD3-CD4-CD8- (triple negative, TN) thymocytes and in fetal thymocytes. The huTAC+, in contrast to the huTAC- subpopulation of the CD44+CD25+ TN cells, was unexpectedly found not to express the pTalpha-gene. Still the huTAC+CD44+CD25+ TN cells reconstituted the development of both alphabeta and gammadelta lineage cells equally efficiently as the pTalpha-expressing huTAC- fraction, demonstrating that this pTalpha-negative subpopulation contained precursors for both T-cell lineages. Single cell reverse transcription-polymerase chain reaction (RT-PCR) experiments demonstrated that also in normal mice only a fraction of CD44+CD25+ and CD44-CD25+ TN cells expressed this gene. Taken together, these data indicate that huTAC transgene expression revealed a truly pTalpha-negative fraction of the CD44+CD25+ TN cells. The observation that not all precursors in the CD25+ TN population express the pTalpha-gene has important implications for the understanding of early T-cell development and T-cell lineage commitment.

Thymic epithelial cells mediate a Bcl-2-independent protection of single-positive thymocytes from dexamethasone-induced apoptosis

Intrathymic maturation of thymocytes is essential for the proper formation of T-cell repertoire. This process involves two major biochemical pathways, one initiated by the recognition of MHC/peptide by the T-cell receptor and the other mediated by glucocorticoids. These hormones seem to affect thymocyte maturation by increasing the threshold of TCR-mediated positive and negative selection, and by inducing apoptosis of nonselected thymocytes. We have previously reported that an SV40-immortalized murine thymic epithelial cell line, namely 2BH4, was able to protect thymocytes from dexamethasone-induced apoptosis. Here we show that this protection is independent of cell-to-cell contact and does not seem to involve a Bcl-2-mediated resistance, since incubation of thymocytes with 2BH4 cells or its supernatant does not interfere with the levels of this antiapoptotic molecule. The protection conferred by 2BH4 cells, or by a primary culture of thymic stromal cells, is specific for the CD4(+)CD8(-) and CD4(-)CD8(+) single-positive thymocytes, whereas the broad-spectrum caspase inhibitor z-VAD-fmk blocks apoptosis induced by dexamethasone in all thymocyte subpopulations. Our results suggest that positively selected single-positive thymocytes are still susceptible to glucocorticoid-induced apoptosis but are protected from it through the action of a heat-stable protein(s) released by thymic stromal cells.

Blood gammadelta T cells and gammadelta TCR V gene specificities in a single missense mutation (L-->Q271) in the common gamma chain gene

The numbers of blood CD4+, CD8+, and CD4-CD8- T cells bearing alphabeta T-cell receptor (TCR) or gammadelta TCR molecules in males with a single missense mutation (L-->Q271) in the common gamma chain gene (gamma(c)) were investigated by flow cytometry. Virtually all XCIDL-->Q271 blood T cells that were CD4+ or CD8+ displayed alphabeta TCR but no gammadelta TCR. In contrast, CD4-CD8- T cells from affected males usually displayed gammadelta TCR, but no alphabeta TCR. The gammadelta TCR specificities were also studied. Except for the oldest subject, there was a direct relationship between blood CD3+ T cells that displayed gammadelta TCR and Vgamma9 and Vdelta2a specificities. Relative frequencies of CD3+ blood T cells that were Vgamma9+ or Vdelta2a+ were inversely related to age. In the oldest patient, the only detected gammadelta TCR specificity was Vdelta1. Thus, in contrast to mice with no gamma(c), XCIDL-Q271 blood T cells that bear gammadelta TCR with Vgamma9/Vdelta2a specificities develop but then decline in late childhood and thereafter. TCR with the Vdelta1 specificity then appear in older survivors with XCIDL-->Q271.

TCRA gene rearrangement in immature thymocytes in absence of CD3, pre-TCR, and TCR signaling

During thymocyte differentiation, TCRA genes are massively rearranged only after productively rearranged TCRB genes are expressed in association with pTalpha and CD3 complex molecules within a pre-TCR. Signaling from the pre-TCR via the CD3 complex is thought to be required to promote TCRA gene accessibility and recombination. However, alphabeta(+) thymocytes do develop in pTalpha-deficient mice, showing that TCRalpha-chain genes are rearranged, either in CD4(-)CD8(-) or CD4(+)CD8(+) thymocytes, in the absence of pre-TCR expression. In this study, we analyzed the TCRA gene recombination status of early immature thymocytes in mutant mice with arrested thymocyte development, deficient for either CD3 or pTalpha and gammac expression. ADV genes belonging to different families were found rearranged to multiple AJ segments in both cases. Thus, TCRA gene rearrangement is independent of CD3 and gammac signaling. However, CD3 expression was found to play a role in transcription of rearranged TCRalpha-chain genes in CD4(-)CD8(-) thymocytes. Taken together, these results provide new insights into the molecular control of early T cell differentiation.

Somatic activation of beta-catenin bypasses pre-TCR signaling and TCR selection in thymocyte development

Mutation or ablation of T cell factor 1 and lymphocyte enhancer factor 1 indicated involvement of the Wnt pathway in thymocyte development. The central effector of the Wnt pathway is beta-catenin, which undergoes stabilization upon binding of Wnt ligands to frizzled receptors. We report here that conditional stabilization of beta-catenin in immature thymocytes resulted in the generation of single positive T cells that lacked the alpha beta TCR and developed in the absence of pre-TCR signaling and TCR selection. Although active beta-catenin induced differentiation in the absence of TCRs, its action was associated with reduced proliferation and survival when compared to developmental changes induced by the pre-TCR or the alpha beta TCR.

Differential roles of cytokine receptors in the development of epidermal gamma delta T cells

IL-7 and IL-15 play important roles in gammadelta T cell development. These receptors transmit proliferation and/or survival signals in gammadelta T cells. In addition, the IL-7R promotes recombination and transcription in the TCR gamma locus. To clarify the role of the cytokine receptors in the development of epidermal gammadelta T cells, we introduced a Vgamma3/Vdelta1 TCR transgene, derived from Thy-1+ dendritic epidermal T cells (DETC), into IL-7Ralpha-deficient mice, and we found that they partly rescued gammadelta T cells in the adult thymus but not in the spleen. Introduction of an additional Bcl-2 transgene had a minimal effect on gammadelta T cells in the adult thymus of these mice. In contrast to the adult thymus, the introduction of the Vgamma3/Vdelta1 TCR transgene into IL-7Ralpha-/- mice completely restored Vgamma3+ T cells in the fetal thymus and DETC in the adult skin. On the contrary, the same Vgamma3/Vdelta1 TCR transgene failed to rescue DETC in the skin of IL-2Rbeta-deficient mice, even with the additional Bcl-2 transgene. These results suggest that the IL-2/IL-15R, rather than the IL-7R, plays an essential role in proliferation and survival of DETC in the fetal thymus and the skin. In contrast, the IL-7R is probably essential in the induction of V-J recombination of TCRgamma genes. Thus, this study proves that IL-7R and IL-2/IL-15R serve differential functions in epidermal gammadelta T cell development.